Combustion of fossil fuels produces a number of undesirable pollutants including oxides of nitrogen (NOx) and carbon monoxide (CO). Environmental degradation attributable to NOx and CO has become a matter of increasing concern, leading to intense interest in suppressing NOx and CO formation in fuel burning devices.
One of the principal strategies for inhibiting NOx formation is to burn a fuel-air mixture that is both stoichiometrically lean and thoroughly blended. Lean stoichiometry and thorough blending keep the combustion flame temperature uniformly low--a prerequisite for inhibiting NOx formation. One type of fuel injector that produces a lean, thoroughly blended fuel-air mixture is a tangential entry injector. Examples of tangential entry fuel injectors for gas turbine engines are provided in U.S. Pat. Nos. 5,307,643, 5,402,633, 5,461,865 and 5,479,773, all of which are assigned to the assignee of the present application. These fuel injectors have a mixing chamber radially outwardly bounded by a pair of cylindrical-arc, offset scrolls. Adjacent ends of the scrolls define air admission slots for admitting air tangentially into the mixing chamber. An array of fuel injection passages extends axially along the length of each slot. A fuel injector centerbody extends aftwardly from the forward end of the injector to define the radially inner boundary of the mixing chamber. The centerbody may include provisions for introducing additional fuel into the mixing chamber. During engine operation, a stream of combustion air enters the mixing chamber tangentially through the air admission slots while fuel is injected into the air stream through each of the fuel injection passages. The fuel and air swirl around the centerbody and become intimately and uniformly intermixed in the mixing chamber. The fuel-air mixture flows axially aftwardly and is discharged into an engine combustion chamber where the mixture is ignited and burned. The intimate, uniform premixing of the fuel and air in the mixing chamber inhibits NOx formation by ensuring a uniformly low combustion flame temperature.
Despite the many merits of the tangential entry injectors referred to above, they are not without certain shortcomings. One shortcoming is that the fuel-air mixture in the mixing chamber can encourage the combustion flame to migrate into the mixing chamber where the flame can quickly damage the scrolls and centerbody. A second shortcoming is related to the flame's tendency to be spatially and temporally unstable even if it remains outside the mixing chamber. This flame instability, which is formally known as an aero-thermal acoustic resonance, is manifested by fluctuations in the position of the flame and accompanying, low frequency pressure oscillations. The repetitive character of the pressure oscillations can stress the combustion chamber, compromising its structural integrity and reducing its useful life. An improved tangential entry fuel injector that addresses these shortcomings is described in U.S. patent application Ser. No. 08/991,032 filed on Dec. 15, 1997 and assigned to the assignee of the present application. The disclosed injector includes a unique array of fuel injection passages for injecting fuel into the tangentially entering airstream, and an aerodynamically contoured centerbody featuring a bluff tip aligned with the injector's discharge plane. Fuel and air discharge openings extend through the centerbody tip for discharging jets of fuel and air into the combustion chamber at the injector discharge plane. The passage array and centerbody shape cooperate to resist flame ingestion and disgorge any flame that becomes ingested. The bluff, fueled tip provides a surface for anchoring the combustion flame, improving the flame's stability and further counteracting any tendency of the flame to migrate into the mixing chamber. The air flowing through the air discharge openings in the tip helps to support combustion and cool the tip.
Although the improved injector addresses the problems of flame stability and flame ingestion, the durability of the injector may be inadequate for extended, trouble free service. Because the centerbody tip is directly exposed to the anchored combustion flame, the tip operates at temperatures high enough to limit its useful life. The velocity and quantity of cooling air flowing through the tip passages could be increased to improve the temperature tolerance of the tip. However increasing the cooling air velocity tends to destabilize the combustion flame by weakening its propensity to remain attached to the tip. Increasing the cooling air quantity is also undesirable because the cooling air not only cools the tip but also reduces the flame temperature. Although low flame temperature suppresses NOx formation, a flame that is too cool also inhibits a combustion reaction that converts carbon monoxide to more environmentally benign carbon dioxide. Thus, although NOx emissions may be satisfactory, CO emissions may be unacceptably high.
What is sought is an advanced, premixing fuel injector that balances the conflicting demands of good durability and superior flame stability without increasing CO emissions.